Solicit assistance from a reputable local bike shop and/or bike fitter if you are totally unsure about frame geometry / sizing and what will fit you. Your style of riding will influence final fit, but the task of identifying frame size is largely based on your physical dimensions and level of flexibility. Also, don't gorget that saddle selection, saddle setback, stem length, handlebar width, and crank length will also have a significant influence on final fit and comfort. And after all that is said and done, you have shoe selection, cleat placement, and orthotics to figure out. There is a lot more to comfort and efficiency on the bike than most people think. Good luck! The process of getting a new bike is always exciting.